Vaccinia virus has two copies of a gene which has a 50-amino acid predicted sequence homology with the receptor binding site of epidermal growth factor (EGF) and transforming growth factor (TGF-Alpha). A 25 kilodalton glycoprotein (vaccinia virus growth factor [VGF] that can bind to EGF receptors) has been purified from media of virus infected BS-C-1 cells. When a mutant virus (VGF-) was constructed by removing both copies of the sequence encoding the putative EGF receptor binding site, the EGF receptor binding activity in infected cells was abolished. Inoculations of the VGF- mutant into BALB/cByJ mice by the intracranial route resulted in an LD50 which was 100-fold higher than the wild-type (WT) virus. A similar attenuated phenotype was observed with the mutant on intradermal inoculation of the skin of the New Zealand white rabbit. The growth of this mutant was extensively investigated in BSC-1 and A431 (high density of EGF receptors on cells) cell lines, and on the chlorio-allantoic membrane (CAM) of the chicken egg. BS-C-1 cells maintained either in high or low fetal calf serum supported the replication of the VGF- mutant to the same degree as WT. In A431 cells, the mutant produced a similar yield of progeny virus as WT in the growth conditions examined, although the plaque morphology of the mutant was markedly different from the WT. The WT virus showed a piling up of cells around the foci of infection. The mutant, on the other hand, produced a typical plaque indistinguishable from that observed on BS-C-1 cells. On the CAM of the egg, both WT and mutant virus formed pocks with approximately the same efficiency. The WT, but not the mutant, induced extensive ectoderm and endoderm proliferation. WT lesions were on the whole slightly larger than the mutant, and contained greater amounts of infectivity. Both viruses induce a similar inflammatory response in the host which was observed at 40 hours post inoculation.